68-557: Biarmosuchia is an extinct clade of non-mammalian synapsids from the Permian . Biarmosuchians are the most basal group of the therapsids . They were moderately-sized, lightly built carnivores, intermediate in form between basal sphenacodont " pelycosaurs " and more advanced therapsids. Biarmosuchians were rare components of Permian ecosystems, and the majority of species belong to the clade Burnetiamorpha, which are characterized by elaborate cranial ornamentation. The biarmosuchian skull
136-806: A bony arch beneath each; this accounts for the name "synapsid". The distinctive temporal fenestra developed about 318 million years ago during the Late Carboniferous period, when synapsids and sauropsids diverged, but was subsequently merged with the orbit in early mammals. The basal amniotes ( reptiliomorphs ) from which synapsids evolved were historically simply called "reptiles". Therefore, stem group synapsids were then described as mammal-like reptiles in classical systematics, and non- therapsid synapsids were also referred to as pelycosaurs or pelycosaur- grade synapsids . These paraphyletic terms have now fallen out of favor and are only used informally (if at all) in modern literature, as it
204-431: A secondary palate , separating the mouth and nasal cavity . In early synapsids, a secondary palate began to form on the sides of the maxilla , still leaving the mouth and nostril connected. Eventually, the two sides of the palate began to curve together, forming a U shape instead of a C shape. The palate also began to extend back toward the throat, securing the entire mouth and creating a full palatine bone . The maxilla
272-435: A calcified layer, as most modern reptiles and monotremes do. This may also explain why there is no fossil evidence for synapsid eggs to date. Because they were vulnerable to desiccation, secretions from apocrine -like glands may have helped keep the eggs moist. According to Oftedal, early synapsids may have buried the eggs into moisture laden soil, hydrating them with contact with the moist skin, or may have carried them in
340-464: A cascade of biotic response. Compared to their pelycosaur ancestors, Leucocephalus and other early therapsids are distinguished by more vertical (mammal like) leg positioning beneath their bodies, larger temporal fenestra and increased jaw complexity and power. Like other burnetiids, Leucocephalus skulls exhibit numerous distinguished bony protuberances and bosses giving it a bumpy appearance. These include paired supratemporal "horns" formed by
408-664: A cat. Most Jurassic and Cretaceous cynodonts were herbivorous , though some were carnivorous . The family Tritheledontidae , which first appeared near the end of the Triassic, was carnivorous and persisted well into the Middle Jurassic . The other, Tritylodontidae , first appeared at the same time as the tritheledonts, but was herbivorous. This group became extinct at the end of the Early Cretaceous epoch. Dicynodonts are generally thought to have become extinct near
476-474: A clade containing both the traditional therapsid families and mammals. Although Synapsida and Therapsida include modern mammals, in practical usage, those two terms are used almost exclusively when referring to the more basal members that lie outside of Mammaliaformes . Synapsids evolved a temporal fenestra behind each eye orbit on the lateral surface of the skull. It may have provided new attachment sites for jaw muscles. A similar development took place in
544-482: A horny overlay, like those found in modern crocodiles and turtles . These differed in structure from the scales of lizards and snakes , which are an epidermal feature (like mammalian hair or avian feathers). Recently, skin impressions from the genus Ascendonanus suggest that at least varanopsids developed scales similar to those of squamates . It is currently unknown exactly when mammalian characteristics such as body hair and mammary glands first appeared, as
612-517: A jaw joint in which one of the smaller bones of the lower jaw, the articular, makes a connection with a bone of the cranium called the quadrate bone to form the articular-quadrate jaw joint. In forms transitional to mammals, the jaw joint is composed of a large, lower jaw bone (similar to the dentary found in mammals) that does not connect to the squamosal, but connects to the quadrate with a receding articular bone. Over time, as synapsids became more mammalian and less 'reptilian', they began to develop
680-492: A moist pouch, similar to that of monotremes ( echidnas carry their eggs and offspring via a temporary pouch ), though this would limit the mobility of the parent. The latter may have been the primitive form of egg care in synapsids rather than simply burying the eggs, and the constraint on the parent's mobility would have been solved by having the eggs "parked" in nests during foraging or other activities and periodically be hydrated, allowing higher clutch sizes than could fit inside
748-546: A natural group or an assemblage that had in common only shared primitive characteristics. They thought that Phthinosuchus was too poorly known to tell if it also belonged, but considered Eotitanosuchus a more advanced form. Denise Sigogneau-Russell (1989) erected the infraorder Biarmosuchia to include the families Biarmosuchidae, Hipposauridae and Ictidorhinidae, distinct from Eotitanosuchia and Phthinosuchia. Ivakhnenko (1999) argued that Biarmosuchus tener , Eotitanosuchus olsoni , and Ivantosaurus ensifer , all known from
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#1732787867556816-746: A partial skull roof including the dorsal margin of orbits and parietal foramen of an unnamed burnetiid from the upper Permian of Tanzania, and Sidor et al . (2014) noted the presence of a burnetiid in the middle Permian of Zambia. Other Biarmosuchia include Biarmosuchus from Russia, Hipposaurus , Herpetoskylax , Ictidorhinus and Lycaenodon from South Africa, and Wantulignathus from Zambia. Biarmosuchus Hipposaurus Ictidorhinus Herpetoskylax Lycaenodon Lemurosaurus Bullacephalus Pachydectes Lobalopex Lophorhinus Niuksenitia Proburnetia Burnetia Paraburnetia Leucocephalus Lende Biarmosuchians are typically considered
884-575: A pouch (or pouches) at once, and large eggs, which would be cumbersome to carry in a pouch, would be easier to care for. The basis of Oftedal's speculation is the fact that many species of anurans can carry eggs or tadpoles attached to the skin, or embedded within cutaneous "pouches" and how most salamanders curl around their eggs to keep them moist, both groups also having glandular skin. The glands involved in this mechanism would later evolve into true mammary glands with multiple modes of secretion in association with hair follicles. Comparative analyses of
952-531: A single skull was found which was located in 2012 at a farm called Amandelboom in Northern Cape Province . It was found on a slope with strata that hosted a tetrapod fossil assemblage to the lower Tropidostoma AZ by a local sheep herder who then hung it on a fence on his farm. Although the skull was outside of its site of burial, strata stuck to it was verified to match that of the adjacent cliff section. The biostratigraphic occurrence of
1020-555: A skull 60 centimetres (24 in) in length. Currently the most representative group of the Biarmosuchia, the Burnetiamorpha, comprise ten genera: Bullacephalus , Burnetia , Lemurosaurus , Lobalopex , Lophorhinus , Paraburnetia , and Pachydectes from South Africa, Niuksenitia and Proburnetia from Russia, and Lende (MAL 290) from Malawi. In addition, Sidor et al . (2010) recently described
1088-444: A stapes. The malleus is derived from the articular (a lower jaw bone), while the incus is derived from the quadrate (a cranial bone). Mammalian jaw structures are also set apart by the dentary-squamosal jaw joint . In this form of jaw joint, the dentary forms a connection with a depression in the squamosal known as the glenoid cavity . In contrast, all other jawed vertebrates, including reptiles and nonmammalian synapsids, possess
1156-617: A ton or more in weight (e.g.: Moschops ). After flourishing for many millions of years, these successful animals were all but wiped out by the Permian–Triassic mass extinction about 250 mya, the largest known extinction in Earth's history , possibly related to the Siberian Traps volcanic event. Only a few therapsids went on to be successful in the new early Triassic landscape; they include Lystrosaurus and Cynognathus ,
1224-404: Is a diverse group of tetrapod vertebrates that includes all mammals and their extinct relatives. It is one of the two major clades of the group Amniota , the other being the more diverse group Sauropsida (which includes all extant reptiles and birds ). Unlike other amniotes, synapsids have a single temporal fenestra , an opening low in the skull roof behind each eye socket , leaving
1292-613: Is a genus of biarmosuchian belonging to the family Burnetiidae dating to the Wuchiapingian ( Late Permian ). It was found in the Tropidostoma Assemblage Zone of the Main Karoo Basin of South Africa . It is a monotypic taxon which contains one only species, Leucocephalus wewersi . The genus name Leucocephalus is derived from Greek. Leucos, meaning white; kephalos, meaning skull, as
1360-422: Is also closed completely. In fossils of one of the first eutheriodonts , the beginnings of a palate are clearly visible. The later Thrinaxodon has a full and completely closed palate, forming a clear progression. In addition to the glandular skin covered in fur found in most modern mammals, modern and extinct synapsids possess a variety of modified skin coverings, including osteoderms (bony armor embedded in
1428-551: Is believed to have been in species that lived more than 300 million years ago. However, Late Permian coprolites from Russia and possibly South Africa showcase that at least some synapsids did already have pre-mammalian hair in this epoch. These are the oldest impressions of hair-like structures on synapsids. Early synapsids, as far back as their known evolutionary debut in the Late Carboniferous period, may have laid parchment-shelled (leathery) eggs, which lacked
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#17327878675561496-489: Is incorrect under the new definition of "reptile", so they are now referred to as stem mammals , proto-mammals , paramammals or pan-mammals . Most lineages of pelycosaur-grade synapsids were replaced by the more advanced therapsids, which evolved from sphenacodontoid pelycosaurs, at the end of the Early Permian during the so-called Olson's Extinction . Synapsids were the largest terrestrial vertebrates in
1564-452: Is known for most species. However, they were moderately diverse and there were multiple contemporary species in some ecosystems. All were predators similar to gorgonopsians and therocephalians , though they were generally not apex predators. [REDACTED] Synapsids Theropsida ( Seeley , 1895) " Pelycosauria " (Cladistically including therapsids) Synapsida
1632-415: Is now known that all extant reptiles are more closely related to each other and birds than to synapsids, so the word "reptile" has been re-defined to mean only members of Sauropsida or even just an under-clade thereof. In a cladistic sense, synapsids are in fact a monophyletic sister taxon of sauropsids, rather than a part of the sauropsid lineage. Therefore, calling synapsids "mammal-like reptiles"
1700-424: Is now southern Africa may have been the area of origin for burnetiamorphs. During the period Leucocephalus lived, what is considered the most extensive mass extinction in the history of the earth was occurring, which caused over 80% of the all Earth's species to go extinct. The cause of this end-Permian mass extinction is hypothesized to be climate change induced by volcanic CO 2 degassing which lead to
1768-445: Is very similar to the sphenacodontid skull, differing only in the larger temporal fenestra (although these are still small relative to later therapsids), slightly backward-sloping occiput (the reverse of the pelycosaur condition), reduced number of teeth, and single large canine teeth in both upper and lower jaws, and other features (Carroll 1988 pp. 370, Benton 2000 p. 114). In later specialised Biarmosuchia, these resemble
1836-561: The Cynodontia are also hypothesized to have had fur or a fur-like covering based on their inferred warm-blooded metabolism. While more direct evidence of fur in early cynodonts has been proposed in the form of small pits on the snout possibly associated with whiskers , such pits are also found in some reptiles that lack whiskers. There is evidence that some other non-mammalian cynodonts more basal than Castorocauda , such as Morganucodon , had Harderian glands , which are associated with
1904-589: The Early Triassic . Synapsid population and diversity were severely reduced by the Capitanian mass extinction event and the Permian–Triassic extinction event , and only two groups of therapsids, the dicynodonts and eutheriodonts (consisting of therocephalians and cynodonts ) are known to have survived into the Triassic . These therapsids rebounded as disaster taxa during the early Mesozoic, with
1972-526: The Ezhovo locality, Ocher Faunal Assemblage , are actually the same species. Even if these taxa are shown to be distinct, Ivakhnenko's paper indicates that Eotitanosuchus and Biarmosuchus are very similar animals. Ivakhnenko also relocates the family Eotitanosuchidae to the order Titanosuchia, superorder Dinocephalia . Benton 2000 and 2004 gives the Biarmosuchia the rank of suborder. Biarmosuchians were rare components of their ecosystems; only one specimen
2040-647: The Leucocephalus skull discovered was unusually pale. The species epithet wewersi comes from the farm employee who found the skull, Klaus ‘Klaasie’ Wewers. Biarmosuchians are a group of some of the earliest therapsids , a group of synapsids including mammals and their ancestors. The skull of Leucocephalus was found in the Tropidostoma Assemblage Zone (Tropidostoma) of the Main Karoo Basin of South Africa. Only
2108-598: The Olenekian age, an early representative of which was Cynognathus . Unlike the dicynodonts, which were large, the cynodonts became progressively smaller and more mammal-like as the Triassic progressed, though some forms like Trucidocynodon remained large. The first mammaliaforms evolved from the cynodonts during the early Norian age of the Late Triassic, about 225 mya. During the evolutionary succession from early therapsid to cynodont to eucynodont to mammal,
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2176-454: The Permian period (299 to 251 mya ), rivalled only by some large pareiasaurian parareptiles such as Scutosaurus . They were the dominant land predators of the late Paleozoic and early Mesozoic , with eupelycosaurs such as Dimetrodon , Titanophoneus and Inostrancevia being the apex predators during the Permian, and theriodonts such as Moschorhinus during
2244-405: The canines , molars , and incisors . The trend towards differentiation is found in some labyrinthodonts and early anapsid reptilians in the form of enlargement of the first teeth on the maxilla , forming a sort of protocanines. This trait was subsequently lost in the diapsid line, but developed further in the synapsids. Early synapsids could have two or even three enlarged "canines", but in
2312-443: The diapsids , which evolved two rather than one opening behind each eye. Originally, the openings in the skull left the inner cranium covered only by the jaw muscles, but in higher therapsids and mammals, the sphenoid bone has expanded to close the opening. This has left the lower margin of the opening as an arch extending from the lower edges of the braincase. Synapsids are characterized by having differentiated teeth. These include
2380-405: The squamosals and parietals . In lateral view, a ridge like boss on the nasal extending exteriorly to the prefrontal is present. The snout is notably tall and the supraorbital region contains large triangular bosses. Amongst other members of the burnetiid family, Leucocephalus has some distinct features unique to its genus. This includes a longer and rounder than usual vomerine process which
2448-476: The therocephalians , which only lasted the first 20 million years of the Triassic period. The second were specialised, beaked herbivores known as dicynodonts (such as the Kannemeyeriidae ), which contained some members that reached large size (up to a tonne or more). And finally there were the increasingly mammal-like carnivorous, herbivorous, and insectivorous cynodonts, including the eucynodonts from
2516-708: The 5,500 species of living synapsids, known as the mammals , include both aquatic ( cetaceans ) and flying ( bats ) species, and the largest animal ever known to have existed (the blue whale ). Humans are synapsids, as well. Most mammals are viviparous and give birth to live young rather than laying eggs with the exception being the monotremes . Triassic and Jurassic ancestors of living mammals, along with their close relatives, had high metabolic rates. This meant consuming food (generally thought to be insects) in much greater quantity. To facilitate rapid digestion , these synapsids evolved mastication (chewing) and specialized teeth that aided chewing. Limbs also evolved to move under
2584-399: The biosphere and left vacant niches open to be filled by newly evolved taxa. In non-mammaliaform synapsids, those taxa that gave rise to rapidly diversifying lineages have been both small and large in body size, although after the Late Triassic, progenitors of new synapsid lineages have generally been small, unspecialised generalists. The earliest known synapsid Asaphestera coexisted with
2652-745: The body instead of to the side, allowing them to breathe more efficiently during locomotion. This helped make it possible to support their higher metabolic demands. Below is a cladogram of the most commonly accepted phylogeny of synapsids, showing a long stem lineage including Mammalia and successively more basal clades such as Theriodontia, Therapsida and Sphenacodontia: † Caseasauria [REDACTED] † Varanopidae [REDACTED] † Ophiacodontidae [REDACTED] † Edaphosauridae [REDACTED] † Sphenacodontidae [REDACTED] † Biarmosuchia [REDACTED] † Dinocephalia [REDACTED] † Anomodontia [REDACTED] † Gorgonopsia [REDACTED] Leucocephalus Leucocephalus
2720-474: The branch within which mammals evolved, and stem mammals, (previously known as pelycosaurs ), comprising the other six more primitive families of synapsids. Stem mammals were all rather lizard-like, with sprawling gait and possibly horny scutes , while therapsids tended to have a more erect pose and possibly hair, at least in some forms. In traditional taxonomy, the Synapsida encompasses two distinct grades :
2788-466: The dicynodont Lystrosaurus making up as much as 95% of all land species at one time, but declined again after the Smithian–Spathian boundary event with their dominant niches largely taken over by the rise of archosaurian sauropsids, first by the pseudosuchians and then by the pterosaurs and dinosaurs . The cynodont group Probainognathia , which includes the group Mammaliaformes , were
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2856-737: The earliest known sauropsid Hylonomus which lived during the Bashkirian age of the Late Carboniferous . It was one of many types of primitive synapsids that are now informally grouped together as stem mammals or sometimes as protomammals (previously known as pelycosaurs ). The early synapsids spread and diversified, becoming the largest terrestrial animals in the latest Carboniferous and Early Permian periods, ranging up to 6 metres (20 ft) in length. They were sprawling, bulky, possibly cold-blooded, and had small brains. Some, such as Dimetrodon , had large sails that might have helped raise their body temperature . A few relict groups lasted into
2924-535: The early synapsids, only two species of small varanopids have been found to possess osteoderms ; fossilized rows of osteoderms indicate bony armour on the neck and back. However, some recent studies have cast doubt on the placement of Varanopidae in Synapsida, while others have countered and lean towards this traditional placement. Skin impressions indicate some early synapsids basal possessed rectangular scutes on their undersides and tails. The pelycosaur scutes probably were nonoverlapping dermal structures with
2992-537: The end of the Triassic period, but there was evidence this group survived, in the form of six fragments of fossil bone that were found in Cretaceous rocks of Queensland , Australia. If true, it would mean there is a significant ghost lineage of Dicynodonts in Gondwana . However, these fossils were re-described in 2019 as being Pleistocene in age, and possibly belonging to a diprotodontid marsupial . Today,
3060-581: The enlarged canines of the Gorgonopsia . The presence of larger jaw-closing muscles (and hence a stronger bite) is indicated by the flaring of the rear of the skull where these muscles were attached. Burnetiamorphs, which made up the majority of biarmosuchian diversity, were characterized by elaborate cranial ornamentation consisting of bumps and bosses. Some burnetiids have a thick domed skull reminiscent of dinocephalians and pachycephalosaur dinosaurs. The vertebrae are also sphenacodontid-like (but lack
3128-472: The evolutionary origin of milk constituents support a scenario in which the secretions from these glands evolved into a complex, nutrient-rich milk long before true mammals arose (with some of the constituents possibly predating the split between the synapsid and sauropsid lines). Cynodonts were almost certainly able to produce this, which allowed a progressive decline of yolk mass and thus egg size, resulting in increasingly altricial hatchlings as milk became
3196-615: The family Kannemeyeriidae) had disappeared by the Rhaetian age, even before the Triassic–Jurassic extinction event that killed off most of the large non-dinosaurian archosaurs . The remaining Mesozoic synapsids were small, ranging from the size of a shrew to the badger-like mammal Repenomamus . During the Jurassic and Cretaceous, the remaining non-mammalian cynodonts were small, such as Tritylodon . No cynodont grew larger than
3264-640: The fossils only rarely provide direct evidence for soft tissues. An exceptionally well-preserved skull of Estemmenosuchus , a therapsid from the Upper Permian, preserves smooth skin with what appear to be glandular depressions, an animal noted as being semi- aquatic . The oldest known fossil showing unambiguous imprints of hair is the Callovian (late middle Jurassic ) Castorocauda and several contemporary haramiyidans , both non-mammalian mammaliaform (see below, however). More primitive members of
3332-471: The four main subclasses of reptiles . However, this notion was disproved upon closer inspection of skeletal remains, as synapsids are differentiated from reptiles by their distinctive temporal openings. These openings in the skull bones allowed the attachment of larger jaw muscles, hence a more efficient bite. Synapsids were subsequently considered to be a later reptilian lineage that became mammals by gradually evolving increasingly mammalian features, hence
3400-407: The grooming and maintenance of fur. The apparent absence of these glands in non-mammaliaformes may suggest that fur did not originate until that point in synapsid evolution. It is possible that fur and associated features of true warm-bloodedness did not appear until some synapsids became extremely small and nocturnal, necessitating a higher metabolism. The oldest examples of nocturnality in synapsids
3468-614: The later Permian but, by the middle of the Late Permian, all had either died off or evolved into their successors, the therapsids. The therapsids, a more advanced group of synapsids, appeared during the Middle Permian and included the largest terrestrial animals in the Middle and Late Permian . They included herbivores and carnivores, ranging from small animals the size of a rat (e.g.: Robertia ), to large, bulky herbivores
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#17327878675563536-507: The latter of which appeared later in the Early Triassic. However, they were accompanied by the early archosaurs (soon to give rise to the dinosaurs ). Some of these archosaurs, such as Euparkeria , were small and lightly built, while others, such as Erythrosuchus , were as big as or bigger than the largest therapsids. After the Permian extinction, the synapsids did not count more than three surviving clades. The first comprised
3604-406: The limbs and tail. Their fingers are elongated, similar to those of bats and colugos and likely sharing similar roles both as wing supports and to hang on tree branches. Within true mammals, aerial locomotion first occurs in volaticotherian eutriconodonts . A fossil Volaticotherium has an exquisitely preserved furry patagium with delicate wrinkles and that is very extensive, "sandwiching"
3672-578: The long neural spines that distinguish Dimetrodon and its kin), but the shoulder and pelvic girdles and the limbs indicate a much more advanced posture. The feet are more symmetrical, indicating that they faced forward throughout the stride, and the phalanges (fingers/toes) are reduced in length so that they are more like that of later synapsids (therapsids and mammals ) (Carroll 1988 pp. 370–1). Biarmosuchians ranged in size from relatively small species with skulls 10–15 cm in length to large species such as Biarmosuchus , which may have had
3740-519: The low-slung stem mammals have given rise to the more erect therapsids, who in their turn have given rise to the mammals. In traditional vertebrate classification, the stem mammals and therapsids were both considered orders of the subclass Synapsida. In phylogenetic nomenclature , the terms are used somewhat differently, as the daughter clades are included. Most papers published during the 21st century have treated "Pelycosaur" as an informal grouping of primitive members. Therapsida has remained in use as
3808-452: The lower jaw of modern and prehistoric reptiles consists of a conglomeration of smaller bones (including the dentary, articular , and others). As they evolved in synapsids, these jaw bones were reduced in size and either lost or, in the case of the articular, gradually moved into the ear, forming one of the middle ear bones: while modern mammals possess the malleus , incus and stapes , basal synapsids (like all other tetrapods) possess only
3876-497: The main lower jaw bone, the dentary, replaced the adjacent bones. Thus, the lower jaw gradually became just one large bone, with several of the smaller jaw bones migrating into the inner ear and allowing sophisticated hearing. Whether through climate change, vegetation change, ecological competition, or a combination of factors, most of the remaining large cynodonts (belonging to the Traversodontidae ) and dicynodonts (of
3944-655: The modern cladistic approach to animal relationships, according to which the only valid groups are those that include all of the descendants of a common ancestor: these are known as monophyletic groups, or clades . Additionally, Reptilia (reptiles) has been revised into a monophyletic group and is considered entirely distinct from Synapsida, falling within Sauropsida , the sister group of Synapsida within Amniota. The synapsids are traditionally divided for convenience, into therapsids , an advanced group of synapsids and
4012-632: The most basal major lineage of therapsids. Biarmosuchia consists of a paraphyletic series of basal biarmosuchians that are fairly typical early therapsids, and the derived clade Burnetiamorpha, characterized by skulls ornamented by horns and bosses. Biarmosuchians were the last of the six major therapsid lineages to be recognized. The majority of biarmosuchians were once considered gorgonopsians. James Hopson and Herbert Richard Barghusen (1986 p. 88) tentatively united Biarmosuchidae and Ictidorhinidae (including Hipposauridae and Rubidginidae) as "Biarmosuchia", but were undecided as to whether they constituted
4080-485: The name "mammal-like reptiles" (also known as pelycosaurs ). These became the traditional terms for all Paleozoic (early) synapsids. More recent studies have debunked this notion as well, and reptiles are now classified within Sauropsida (sauropsids), the sister group to synapsids, thus making synapsids their own taxonomic group. As a result, the paraphyletic terms "mammal-like reptile" and "pelycosaur" are seen as outdated and disfavored in technical literature, and
4148-628: The only synapsids to survive beyond the Triassic, and mammals are the only synapsid lineage that have survived past the Jurassic , having lived mostly nocturnally to avoid competition with dinosaurs. After the Cretaceous-Paleogene extinction wiped out all non-avian dinosaurs and pterosaurs, synapsids (as mammals) rose to dominance once again during the Cenozoic . At the turn of the 20th century, synapsids were thought to be one of
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#17327878675564216-504: The poorly preserved hands and feet and extending to the base of the tail. Argentoconodon , a close relative, shares a similar femur adapted for flight stresses, indicating a similar lifestyle. Therian mammals would only achieve powered flight and gliding long after these early aeronauts became extinct, with the earliest-known gliding metatherians and bats evolving in the Paleocene . Recently, it has been found that endothermy
4284-427: The primary source of nutrition, which is all evidenced by the small body size, the presence of epipubic bones , and limited tooth replacement in advanced cynodonts, as well as in mammaliaforms . Aerial locomotion first began in non-mammalian haramiyidan cynodonts, with Arboroharamiya , Xianshou , Maiopatagium and Vilevolodon bearing exquisitely preserved, fur-covered wing membranes that stretch across
4352-449: The skin), scutes (protective structures of the dermis often with a horny covering), hair or fur, and scale -like structures (often formed from modified hair, as in pangolins and some rodents ). While the skin of reptiles is rather thin, that of mammals has a thick dermal layer. The ancestral skin type of synapsids has been subject to discussion. The type specimen of the oldest known synapsid Asaphestera preserved scales . Among
4420-456: The skull was in the lowermost Tropidostoma Assemblage Zone. Based on previous dating of the surrounding strata, the Leucocephalus skull is estimated to be around 259 Ma. Other Late Permian therapsids have been collected from the same interval and vicinity including dicynodonts ( Pristerodon mackay , Tropidostoma dubium , Diictodon feliceps ), a gorgonopsian, and a pareiasaur. Based on discoveries of early therapsids and biarmosuchians, what
4488-416: The term stem mammal (or sometimes protomammal or paramammal ) is used instead. Phylogenetically , it is now understood that synapsids comprise an independent branch of the tree of life . The monophyly of Synapsida is not in doubt, and the expressions such as "Synapsida contains the mammals" and "synapsids gave rise to the mammals" both express the same phylogenetic hypothesis. This terminology reflects
4556-434: The therapsids, the pattern had settled to one canine in each upper jaw half. The lower canines developed later. The jaw transition is a good classification tool, as most other fossilized features that make a chronological progression from a reptile-like to a mammalian condition follow the progression of the jaw transition. The mandible , or lower jaw, consists of a single, tooth-bearing bone in mammals (the dentary), whereas
4624-536: Was developed as early as Ophiacodon in the late Carboniferous. The presence of fibrolamellar, a specialised type of bone that can grow quickly while maintaining a stable structure, shows that Ophiacodon would have used its high internal body temperature to fuel a fast growth comparable to modern endotherms. Over the course of synapsid evolution, progenitor taxa at the start of adaptive radiations have tended to be derived carnivores. Synapsid adaptive radiations have generally occurred after extinction events that depleted
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